This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent application NO. 2000-287966 filed on Sept. 22, 2000, the entire content of which is incorporated herein by reference.
This invention relates to a production method of a micromachine. More specifically, this invention relates to the production method employing a surface machining.
Conventionally, micromachining sensors (or micromachined sensors) have been produced by production methods for a micromachine employing surface machining. The angular speed sensor having a pectinated electrostatic oscillator is produced as one of the micromachining sensors. To make the pectinated electrostatic oscillator of the angular speed sensor, first of all, a polysilicon layer having the thickness of approximately 2xcexcm is formed on a silicon substrate by thermally diffusing phosphorous after depositing a non-doped polysilicon film which is similar to an amorphous silicon. Next, a micro structure which is to be the pectinated electrostatic oscillator is formed by etching the polysilicon layer.
The aforementioned polysilicon layer has concave portions which are made to be filled by another polysilicon in order to form support beams of the pectinated electrostatic oscillator. If the thickness of polysilicon layer becomes thicker from 2xcexcm or above thickness in order to enlarge the electrostatic capacity of the pectinated electrostatic oscillator, an inner stress is generated from the concave portion in the polysilicon layer. The inner stress causes the pectinated electrostatic oscillator (formed within the polysilicon layer) to be attached to the silicon substrate, whereby the pectinated electrostatic oscillator cannot be floated from the silicon substrate.
The present invention is made to overcome the aforementioned drawback. The present invention provides a production method of the micromachine which reduces the inner stress generated in the concave portion thereof when the concave portion is made to be smooth and flat by filling the polysilicon layer having a predetermined thickness.
In accordance with the first aspect of the present invention, a production method of a micromachine includes a polysilicon film forming step for flattening grooves defined in a sacrificial layer formed on a surface of a silicon substrate by overlaying a polysilicon layer. The production method of the micromachine includes a first processing step for filling the grooves by adding a lower laid portion of the polysilicon layer onto the sacrificial layer, where the lower laid portion has a thickness of more than 0.625 times as large as a maximum width of the grooves. The production method of the micromachine further includes a second processing step for forming the polysilicon layer with a predetermined thickness by adding a upper laid portion of the polysilicon layer onto the lower laid portion, the upper laid portion being formed by depositing polysilicon which has the same impurity concentration as the lower laid portion has.
In accordance with the second aspect of the present invention, a production method of a micromachine modifies the method of the first aspect, wherein the first processing step adds the lower laid portion onto the sacrificial layer by thermal anealing after repeating a deposition of the polysilicon and an ion implantation with a low pressure chemical vapor deposition.
In accordance with the third aspect of the present invention, a production method of a micromachine modifies the method of the first aspect, wherein a micro structure floating apart from the silicon substrate is formed in the polysilicon layer.
In accordance with the fourth aspect of the present invention, wherein the micro structure is an oscillator of a micromachine sensor.
Thus, constructed present invention has the following advantages: At the first processing step, the grooves defined in the sacrificial layer are filled or flattened by overlaying the lower laid portion of the polysilicon layer which has the thickness of more than 0.625 times as large as a maximum width of the grooves, which makes the inner stress rarely generated in the lower laid portion within the grooves. At the second processing step, the polysilicon layer having the predetermined thickness is formed of the lower laid portion and the upper laid portion of the polysilicon layer by overlaying the upper laid portion on the lower laid portion.
At this time, the overlaid upper laid portion has the same impurity concentration as the lower laid portion has, which prevents the inner stress form generating in the upper laid portion. Therefore the polysilicon layer formed by the lower laid portion and the upper laid portion thereof can be in a stable state, whereby the direction of crystal face of the polysilicon layer generally indicates (110) plane direction. In addition, at the first processing step, the lower laid portion is formed on the sacrificial layer by thermal anealing after repeating a deposition of the polysilicon and an ion implantation under a low pressure chemical vapor deposition, which enables the lower laid portion to be formed by fine crystals, so that the inner stress resulted from the grooves rarely generated.
The production method of the micromachine of the present invention enables the polysilicon layer to be thickened to the predetermined level, thereby reducing the generation of the inner stress, with the result that the micro structure which has the thickness such as previously could not be formed by the surface machining can be formed in floating on the silicon substrate. When an oscillator of the micromachined sensor is constructed by the above micro structure capable of having sufficient electrostatic capacity, the production employed the surface machining increases the output power of the micromachine.